The influence of climatic, hydrologic, and soil factors on evapotranspiration rates of Tamarisk (Tamarix pentandra Pall.)

Abstract

In the arid southwestern United States, where water is a limiting factor in agricultural and industrial development, a sizeable portion of the annual precipitation may be lost through evapotranspiration. In Arizona such losses account for approximately 95 per cent of the annual precipitation. Tamarisk (Tamarix pentandra Pall.) is estimated to occupy over one million acres of the flood plains and streambanks in the southwest. Although reported to use a large quantity of water, accurate estimates of evapotranspiration are unknown. Evapotranspiration processes are complex and depend on many interrelationships of the soil-plantatmosphere system. Although, water use by tamarisk has been intensively studied, evapotranspiration measurements under different climatic and hydrologic conditions are not available. The evapotranspiration tent was selected to measure evapotranspiration rates of tamarisk under varying climatic and hydrologic conditions. Intensive investigations of the enclosure effect of the tent were performed. Modifications of the tent reduced serious enclosure effects of the original tent. Evapotranspiration rates measured by the tent agreed favorably with rates computed by Penman's equation. Evapotranspiration rates for an area where the water table depth was approximately 20-feet was greater than an area where the Water table depth was 14-feet. This deviation, which may be attributed to salinity, led to a laboratory investigation of the effects of salinity on transpiration rates of tainarisk. An intensive laboratory study was conducted to determine the effect of salinity on transpiration rates of tamarisk at different vapor pressure deficits. Results indicated that the effect of salinity is dependent on vapor pressure deficit. Transpiration rates were linearily related to vapor pressure deficits at low salinity levels, but a curvilinear relationship was obtained at high salinity levels. An estimate of saturation deficit of the mesophyll cells was determined by extrapolation of transpiration and vapor pressure deficit relationships. These data indicate minimial increases in salt concentrations in the stomatal cavities as indicated by small increases in the mesophyll saturation deficits as the salinity of the root substrate was increased. Root permeability tests were conducted on plants subjects to varying salinity and vapor pressure deficit levels. Results indicated a significant reduction only at the highest salinity and vapor pressure deficit levels.